Generally described, a tangentially fired boiler includes a combustion chamber in which a combination of a flow of a pulverized solid combustible fuel and a flow of air is combusted to generate heat. The heat may be used for any suitable purpose such as for driving a steam turbine for producing steam and the like. The flow of combustible fuel and the flow of air may be introduced along a horizontal plane from multiple locations about the perimeter of the combustion chamber. Specifically, the flow of fuel and the flow of air may be directed tangentially to a focal region along the horizontal plane. This focal region may be substantially concentric within the combustion chamber such that combustion results in the controlled formation of a spiraling fireball.
Overfire air is combustion air that may be tangentially injected into the combustion chamber between the primary firing zone and a furnace outlet. Thorough mixing of the overfire air with the gases in the fireball may achieve low levels of nitrogen oxides, carbon monoxide, and other types of emissions with an overall increase in combustion efficiency.
Such boilers may use a combination of spiral and vertical water tubes positioned about the walls of the combustion chamber. The spiral tubes may benefit from the averaging of the lateral heat absorption variation in each water tube. Moreover, the location of the spiral to vertical transition of the water tubes may have an impact on the number of turns of the spiral wall. Specifically, too many turns may result in an increased pressure drop while too few turns may result in uneven spiral wall tube outlet temperatures. Such uneven outlet temperatures may cause thermal stresses within the tubes and the headers.